Electric heating system



April 16, 1946. E. D. SCHNEIDER ELECTRIC HEATING SYSTEM Filed July 13, 1944 Ihventdr: Elbeht D.Schneider- M is Attorney.

v to all of the heaters.

Patented Apr. 16, 194s .llbertl).8clmeider,Seotla,-N.Y.,a.ignorto General Electric Company, a corporation of New Your Application July 13, 1944, serum. 544.11: 1 cum. (01. 219-) My invention relates to electric heating sys tems, more particularly to electric heating control systems, and has for its object a simple and reliable system for protecting a plurality of electric heating units.

. My invention has special application to he protection of heating apparatus comprising a plurality of heating units which are connected in parallel circuit with each other. Such heating apparatus is used, for example, in air ducts for the purpose of heating air forced through the duct. If.the supply of air should fail or insufficient air be forced over the heaters to carry the heat away, the heaters may become overheated and burn out.

One method of protecting such heaters is to provide a high temperature thermocouple in engagement with the heater operating at the highest temperature, and hence subject to the most severe operating conditions. This thermocouple is arranged to operate switching means in the event of the unit with which it is associated becoming heated to an excessively high temperature, which switching means operates to reduce the power input to the heaters, Thus the thermocouple would be associated with the heater in the hottest part of the air stream. However, a disadvantage of this protective system is that if this heater should burn out, then its temperature would decrease and no protection whatever would be available for the other heaters.

In accordance with my invention, I provide means responsive to the burning out of the high temperature heater for reducing the power input In one form of my invention this comprises anauxiliary resistance connected in series with the highest temperature heater, together with electron discharge device means responsive to the voltage across this auxiliary resistance under normal temperature conditions for normally maintaining the supply of power to the heaters. Inthe event that the highesttemperature heater burns out, the re sulting open circuit reduces the voltage across the auxiliary resistance to zero and the control thereupon operates to reduce the power input to each other one of the heaters.

For a more complete understanding 0! my invention, reference should be had to the accompanying drawing, the single figure of which is a diagrammatic representation of an electric heating system embodying my invention.

Referring to the drawing! have shown my invention in one form as applied to heating apparatus controlled by means of a saturable reactor l connected in circuit with the parallel connected heaters I, l and l between suitable A. C. supp y mains and I. As shown, the heaters I, I and l are of the resistance type. They are suitably mounted in a duct I through which air is forced by a blower I whereby the air is heated in passing over the heaters. Also, the heaters are arranged in spaced relation along the duct in the direction of the air flow which is assumed to be from the right to the left so that the heater 2 is surrounded by the hottest air and hence operates at the highest temperature. 7

The power input to the heaters is adjusted by varying the saturation of a power regulating iron core reactor I. This'is eiiected by varying the amount of direct current passed through a saturating winding I on the reactor by suitable control means, shown as a variable resistor ill connected in circuit with the winding 8 to direct current supply mains. Protection against overtemperature is provided by means of a thermocouple ll mounted on the heater 2 and operating a suitable contact making relay I: which opens a circuit in the event of an excessive temperature of the heater 2 thereby to open a switch I3 in the circuit of the saturating winding 9, this switch ll'constituting operating means for the regulating' reactor I. when the switch I; opens, the

-reactor Iv becomes unsaturated and thereby reduces the power input to the heaters to a low value at which they will not burn out. However, the thermocouple ll does not provide protection for the heaters 3 and l in the event that the heater 2 should burn out because in such case the temperature of the heater 2 would drop and the switch It would remain closed.

To provide protection for the heaters l and 4 in the event that the heater 2 burns out, I conmeet an auxiliary resistance It in series with the heater 2 whereby a voltage drop of small value is provided across the resistance It as long as the heater 2 is in operation and current is passing through it. This voltage across the resistance It I utilize to maintain the switch I: closed. In the event, however, that the heater 2 burns out,

the voltage across the resistance H drops to zero and the switch I! is opened thereby to protect the remaining heaters by reducing the current in each one of the remaining heaters to the predetermined lower value.

The voltage across the resistance H, which in a typical apparatus varied from .3 to 5 volts depending on the current in the heater 2, is stepped up by an iron core transformer ii, the secondary winding oi which is connected to a capacitor or condenser II in series with a rectifier electron discharge device ll whereby the condenser I8 is charged to a predetermined voltage depending upon the voltage supplied by the transformer II.

This voltage across the condenser I I is applied as a "turn on" voltage to the grid of a threeelement electron discharge device II in whose output circuit is an operating coil ll. The condenser voltage causes the device II to pass current and energize the coil is sufliciently to maintain its switch 20 closed. This switch 20 is connected in circuit with the operating coil 2| for the switch It so as to maintain the coil 2! energized and the switch ll closed for normal operation.

As shown, the output voltage for the discharge device II is obtained from A. C. supply mains 22 and I! through a transformer 24 having its secondary connected in the output circuit of the dis charge device It.

The operating coil 2| i also energized from the mains 22 and II, its circuit leading from the main 2! through a normally open switch 25, the switch 20, an interlock switch 26 on the switch It, the coil 2| to the main 2!. The switch 2| is operated by a coil 21 which is connected to the instrument I! and normally energized by the instrument I! but in the event of an excessive temperature in the heater 2, the coil 21 is deenergized and the switch 2! opened.

Included also for adjustment purposes in the input circuit to the discharge device II is an A. C. bias voltage from a resistance 20 con-' nected across the secondary or a transformer 29 condenser ll quickly is discharged through a resistance I! connected in parallel with it.

While I have shown a parflcular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1.A heating system comprising a plurality of electric heaters connected in parallel relation with each other, regulating means for varying the power supplied to said heaters and arranged to provide for a predetermined minimum current in said heaters, means associated with the first one of said heaters for generating a predetermined minimum voltage in response to said minimum current in said first heater, operating means for said regulating means, and means responsive to whose primary is connected to the supply source as imum voltage in response to a predetermined min- In the operation of this protective control apparatus, assuming that the heaters are connected to the supply mains U and I by suitable switching means (not shown), sufilcient current will pass through the resistance II with the regulating reactance' I completely unsaturated to close the switch 26 and maintain it closed after proper adjustment of the-resistance II in circuit with the primary winding of the transformer i8 and adjustment of the resistance 28. The switch it is then closed by closing a push button switch 3| connected across the switch 28 whereby the coil 2| is energized. This connects the coil 8 to the direct current supply mains i'or energization of the coil and the required saturation of the reactor for which the control II is set for the desired power input to the heaters.

said minimum voltage for controlling said operating means to increase the current in said heaters to a predetermined maximum value and for controlling said operating means to reduce the current in each other one of said heaters to a predetermined low value in the event of failure of said first heater reducing said voltage to a value lower than said minimum.

2. A protective system for heating apparatus including a plurality of heaters connected in parallel relation 'with each other comprising means connected in circuit with a first one of said heaters for generating a predetermined minimum current in said first heater, a normally open switch, means energized when said switch is closed for increasing the power supplied to said heaters to a predetermined maximum value, and

I 40 means responsive to said predetermined minimum comprising a duct for the fluid, a plurality 0i In the adjustment of the control apparatus, the

. resistance ill is adjusted to apply a predetermined voltage to the transformer it. Then the control I0 is adjusted temporarily for zero current in the winding 5, the switch 13 remaining closed. Next the tap on the resistance 28 is moved upward to include an increased voltage in opposition to the voltage across the condenser I! in the input circuit of the device is until the switch Ill opens. Then the tap on the resistance 28 is moved downward until the switch 20 closes and then moved downward slightly more to its final position. Now the opening of the test control push button 32 should cause the switch 20 to open, and reclosure of the button should close the switch 20.

- This assures that when the voltage across the reelectric heaters in said duct arranged in spaced relation with respect to each other in the direction of flow oi said fluid so that a most remote first heater in the direction of how of said fluid is heated to a higher temperature than each other one of said heaters. connections for connecting said heaters in parallel with each other for the supply of electric power there to, power regulating means connected in circuit with said heaters arranged to provide for a predetermined minimum current in said heaters, means connected in circuit with said first heater for producing a predetermined minimum voltage in response to said predetermined minimum current, and operating means for said regulating means responsive to said predetermined minimum voltage for increasing the current supplied to said heaters to a predetermined maximum value and for reducing the current in each other one of said heaters to said predetermined minimum value in the event of failure of said first heater reducing said voltage to a lower value,

4. A heating system for heating a flowing fluid comprising a duct for the fluid, a plurality of heating resistors in said duct arranged in spaced relation with respect to each other in the dlrection of flow of said fiuid so that a most remote first resistor in the direction of flow of said fluid resistance I 4 becomes zero, the charge across the is is heated to a higher temperature than each other asoaevs and for reducing the current in each other one of said resistors to said predetermined minimum value in the event of iailure of said first resistor reducing said voltage to a value lower than said minimum.

5. A heating control system comprising a plurality oi resistor heaters, a reactor, connections for supplying electric power to said resistors in parallel relation with each other and in series with said reactor, said reactor providing for a predetermined minimum current in each of said resistors when said reactor is unsaturated, a control resistance connected in series with one 01' said resistors, a saturating winding for said reactor, connections including current regulating means ior supplying a saturating current to said reactor, a normally open switch in said connections, an operating coil for closing said switch for the supply of saturating current to said winding, connections for energizing said coil including a second normally open switch, a second operating coil for closing said second switch when energized, and means responsive to the voltage across said control resistance produced by said minimum currentior energizing said second operating coil for closure of said second switch thereby to provide for energization of said first operating coil for closure of said first switch and energization oi said saturating winding to increase the power input to said resistors, said second switch opening to deenergize said saturating winding in the event oi. failure oi said one resistor reducing said voltage to a predetermined low value insuflicient for said second operating coil to maintain said second switch closed whereby the current in each other one of said resistors is reduced to said minimum value.

6. A heating system comprising a plurality of electric heaters connected in parallel relation with each other, regulating means for varying the power supplied to said heaters, means responsive to the temperature of one of said heaters for operating said regulating means to decrease the power input to said heaters in the event of an excessive temperature in said one heater, and means responsive to the current in said one heater for controlling said regulating means jointly with said temperature responsive means so as to operate said regulating means in re-' sponse to a predetermined minimum current in said one heater to decrease the current in each other one of said heaters to a predetermined minimum value.

7. A heating system comprising a plurality of electric heaters connected in parallel relation with each other, regulating means for varying the power supplied to said heaters and arranged to provide a predeterminedminimum current in said heaters, a normally open switch for controlling said regulating means to .increase the current in said heaters when said switch is closed, an operating coil for said switch, means responsive to the temperature of one of said heaters for opening and closing the circuit of said coil for operation of said switch to limit the temperatur of said heaters, means responsive to said minimum current in said one of said heaters for producing a predetermined minimum voltage, and switching means responsive to said minimum voltage ior controlling said operating coil jointly with said temperature responsive means thereby to prevent closure of said first switch when said voltage decreases to a value below said minimum value in the event of failure of said one of said heaters whereby said regulating means is caused to limit the current in each other one of said heaters to said minimum value. v

ELBERT D. SCHNEIDER. 

